The present invention relates to a power control circuit for use with a transmitting circuit of a subscriber unit in a radio communication system, for example, a portable telephone system.
PDC (personal digital cellular phone system which will hereinafter simply be referred to as a "portable telephone system") is now available as the standard of radio communication system in Japan. This digital cellular phone system comprises a base station and a mobile station or a subscriber unit, where the transmitted output power of the subscriber unit is controlled by transmitting a transmitted output control signal for controlling a transmitted power from the base station to the subscriber unit. In the specification of the portable telephone system, in the case of a subscriber unit whose transmission power is 0.8 W, it is requested that a transmission output power is controlled at the step of 4 dB in a range of from 0 to -20 dB where the maximum transmission output power is 0 dB.
Japanese laid-open patent publication No. 6-196939 describes a power control circuit for controlling a transmission output power as described above. Circuits shown in FIGS. 1 and 2 are proposed as examples of such a power control circuit.
In the circuit shown in FIG. 1, baseband digital audio data I, Q, which result from A/D-converting audio signals, are supplied through an input terminal 40 to an orthogonal modulator 41, in which they are modulated by using a first local oscillation signal from a first local oscillator 42 as a carrier signal.
A modulated signal from the orthogonal modulator 41 is supplied to a mixer 43 which mixes this modulated signal with a second local oscillation signal supplied thereto from a second local oscillator 42 to provide a carrier signal of 900 MHz band, for example. A transmitted signal from the mixer 43 is supplied through a first variable gain amplifier 45, a divider 46 and a second variable gain amplifier 47 to a power amplifier 48. Further, a transmitted output from the power amplifier 48 is supplied through a directional coupler 49, and an isolator 50 to an antenna 51.
A signal from the divider 46 is supplied through a first detector 52 to a comparator 53. A signal from the directional coupler 49 is supplied to a step-attenuator 54, and a signal from the step-attenuator 54 is supplied through a second detector 55 to the comparator 53. An output from the comparator 53 is supplied to a control unit of the second variable gain amplifier 47.
As a consequence, the gain of the second variable gain amplifier 47 is controlled in such a manner that the level of the signal from the first detector 52 and the level of the signal from the second detector 55 become equal to each other. In this circuit, the first variable gain amplifier 45 is provided in order to correct a fluctuation of an output power of the orthogonal modulator 41. Then, the gain of the first variable gain amplifier 45 is adjusted by a half-fixed signal from a terminal 56 such that the output power of the orthogonal modulator 41 is matched with an optimum operation point with a well-balanced distortion characteristic.
In this circuit, an attenuation amount of the step-attenuator 54 is set in accordance with a transmission output control signal supplied to a terminal 57 from the base station. Then, when a feedback control is effected such that a difference between the level attenuated by the step-attenuator 54 from the output level of the power amplifier 48 and the level of the transmission signal from the mixer 43 becomes constant, the transmission output power supplied to the antenna 51 is controlled in accordance with an attenuation amount set by this step-attenuator 54.
Specifically, a transmission power of the subscriber unit incorporating this circuit is identified by the base station side as a reception level. Then, the base station side transmits a transmission output control signal to the base station such that the transmission output power becomes a lowest level necessary for reception. An attenuation amount of the step-attenuator 54 is set in accordance with the transmission output control signal. Further, the transmission output power supplied to the antenna 51 is controlled in response to the attenuation amount set in the step-attenuator 54, whereby the transmission output power can be set with substantially the same accuracy as that of the step-attenuator 54.
If the attenuation amount, for example, of the step-attenuator 45 is set at the step of 4 dB in a range of 20 dB, then in the case of the subscriber unit whose transmission output power is 0.8 W, the transmission output power can be controlled at the step of 4 dB in a range of from 0 to -20 dB where the maximum transmission output power is 0 dB.
In this manner, the transmission output power of the subscriber unit is controlled in accordance with the transmission output control signal transmitted from the base station to the subscriber unit. Thus, it is possible to prevent another base station from being disturbed by the interference which will occur due to an excessively large transmission level. Also, a consumption of a battery incorporated in the subscriber unit can be lessened, and a life of the battery can be extended.
However, in this circuit, the output supplied from the first detector 52 to the comparator 53 is used as a reference, and should be held constant. As a result, a voltage or the like of the signal that was supplied to the terminal 56 in order to adjust the gain of the first variable gain amplifier 45 should be compensated in temperature, which requires a complex circuit arrangement on the outside.
On the other hand, in the circuit shown in FIG. 2, instead of the signal from the first detector 52 shown in FIG. 1, a fixed reference voltage applied to a terminal 58 is supplied to the comparator 53, while the rest of the arrangement is similar to that of FIG. 1. In this circuit, the gain of the second variable gain amplifier 47 is controlled such that the signal from the second detector 55 and the fixed reference voltage from the terminal 58 become equal to each other.
Therefore, similarly to the circuit shown in FIG. 1, the transmission output power supplied to the antenna 51 is controlled in response to the attenuation amount set in the step-attenuator 54, whereby the transmission output power can be set with substantially the same accuracy as that of the step-attenuator 54. In this case, since the fixed reference voltage is supplied to the comparator 53 from the terminal 58, the signal from the first detector 52 shown in FIG. 2 need not always be constant, the voltage or the like of the signal that was supplied to the terminal 56 in order to adjust the gain of the first variable gain amplifier 45 need not be compensated in temperature or the like.
However, this circuit also needs the first variable gain amplifier 45 in order to correct a fluctuation of the output power of the orthogonal modulator 41 such that the output power of the orthogonal modulator 41 is matched with the optimum operation point with a well-balanced distortion characteristic, and the gain of the first variable gain amplifier 45 has to be adjusted by the signal from the terminal 56. To this end, also in this circuit, the voltage or the like of the signal that was supplied to the terminal 56 in order to adjust the gain of the first variable gain amplifier 45 has to be compensated in temperature, which requires a complex circuit arrangement on the outside.